Osteoclasts are the principal cells that migrate to the potential bone resorption sites and resorb bone under normal and pathological conditions. Our preliminary data has shown that in osteoclasts, deletion of cbl gene impairs osteoclast migration during development. Similarly, the ability of Cbl-/- osteoclasts to reorganize the actin cytoskeleton in response to chemotactic stimuli is also significantly compromised. We have also shown that tyrosine phosphorylation-dependent binding of Cbl to PI3-Kinase, a signaling protein required for osteoclast function, is one of the key requirements to organize the signaling events upon integrin activation. Abolishing the interaction between Cbl and PI3K kinase significantly compromises the bone resorbing capacity of osteoclasts both in vivo and in vitro, indicating that formation of the Cbl-PI3K complex and the subsequent signaling events downstream of this complex are necessary for proper functioning of osteoclasts. Although some progress has been made in understanding the roles of Cbl and PI3K in osteoclast biology the questions that remain to be answered are: what is the mechanism of interaction between the two proteins and what are the downstream events that link this complex to osteoclast function? We have addressed these issues the following specific aims: AIM-1. Determines which tyrosine kinase (Src or Syk) is critical for phosphorylation of the CblY731 downstream of av3 activation and the effect of modulating the phosphorylation of CblY731 on osteoclast migration and bone resorption in vitro. AIM-2. Determines the in vivo impact of Cbl-PI3K complex on skeletal tissue in general and osteoclast function particular by using two different knock-in mouse models in which the Cbl-PI3K interaction is either abolished or constitutively activated. AIM-3. Examines the signaling events downstream of Cbl-PI3K interaction that are important in regulating actin cytoskeleton. The proposed studies will unequivocally demonstrate the interdependence of the events in osteoclast downstream of av3-mediated signaling. The specific aims of the proposal will be accomplished through the application of varieties of approaches and methodologies. This includes in vivo studies with knock-out and knock-in mice, biochemical analysis of cultured cells expressing wild type and mutant proteins and biochemical studies of cells isolated form the genetically modified mice. Successful completion of these aims will demonstrate the positive role played by Cbl in osteoclast function and will increase our understanding of the regulation of bone resorption, potentially identifying novel therapeutic intervention to control bone loss related to osteoporosis arthritis, bone metastasis and periodontal disease. Osteoporosis and other bone-related maladies afflict 150 million Americans and constitute a significant burden to ever-rising health care cost. The research proposed here will help in understanding the basic biology behind bone resorption and will help identifying targets for therapeutic interventions to control bone loss.